Citral is a mixture of aldehydes found in essential oils, specifically lemon grass oil and citronella oil, and is commercially useful as a fragrance for making perfumes, as a flavoring agent, or as an intermediate for other fragrances. Aside from its natural derivation, citral can be produced synthetically.
Citral, as known to those skilled in the art and as used herein, refers to a mixture of geranial and neral. Both the neral and geranial (citral), in turn, have isomer forms known as isocitrals. The isocitrals are low boiling impurities normally found in citral, typically resulting from thermal degradation of the citral.
In one process for producing citral, a thermal rearrangement of beta-pinene yields myrcene, which is an intermediate for many fragrance compounds. Hydrochlorination of the myrcene followed by a displacement reaction utilizing a sodium organic salt, wherein the chloride ions are replaced with an ester group, produces the esters of geraniol and nerol. Subsequent saponification of the esters yields the corresponding alcohols, geraniol and nerol. The alcohols are then oxidized to form the corresponding aldehydes of geranial and neral. The product of this oxidation reaction contains unoxidized nerol and geraniol, neral and geranial (citral), and isomers of the neral and geranial (isocitrals).
Citral may also be produced from petrochemical feedstocks; for example, acetylene, isoprene, or isobutylene. In these reactions, as discussed in U.S. Pat. No. 4,288,636 and Great Britain Patent No. 1,381,587, intermediate materials such as dehydrolinalool and prenal are formed from the petrochemical sources. The intermediate compounds are subsequently converted to citral. While the resultant citral product will not contain nerol and geraniol, it will contain isocitrals.
Adsorptive separation techniques may be used to isolate the citral and isocitral from the unoxidized nerol and geraniol, as is disclosed in U.S. Pat. No. 4,605,783 (Hermann A. Zinnen). However, these adsorptive techniques require both adsorption and desorption of the extract components to and from the absorbent. Furthermore, the adsorptive separation techniques do not address the situation where citral is produced from petrochemical feedstocks, and therefore does not contain unoxidized nerol and geraniol.
A more preferred method for isolation of the citral product from the reaction mixture of the citral process is fractional distillation. However, fractional distillation also has limitations affecting the efficient separation of the citral product from the contaminants. Terpene compounds generally are heat-sensitive, and are subject to degradation unless fractionation is carried out under reduced pressure distillation conditions, thus avoiding excessively high temperatures.
Isocitrals are undesirable because they impart a harsh odor and flavor characteristic to citral. Isocitrals have a lower boiling point than citral. Therefore, this isocitral/citral mixture lends itself to purification through distillation. Since the boiling point of isocitrals is close to that of citral, an efficient selective distillation or fractionation process is required for the separation. The separation is complicated in that isocitrals are formed from citral in an equilibrium process which is directly related to temperature. As a result, attempts to purify citral through fractional distillation to remove isocitrals or other undesirable components results in the formation of additional isocitrals. The amount of isocitrals formed will be directly related to the temperature and the degree of fractionation employed. Thus, the more one attempts to purify citral by fractionation (through the use of more efficient distillation equipment or the use of higher reflux ratios) the more isocitrals will be formed.
Distillation under reduced pressure is helpful in reducing distillation temperatures; however, isocitrals are still formed and high levels of purification (over about 95% citral) are difficult to achieve. Further, such high levels of purity, if achieved by fractional distillation, usually involve a high level of citral conversion to isocitrals, and therefore, the yield of citral is greatly reduced.
Specifically, where citral is being isolated from a mixture or being purified, there are three factors which may affect the purity and yield of citral derived from distillation.
In addition to the heat sensitivity mentioned above, a second factor related to the heat sensitivity of the citral is the equilibrium reaction wherein isocitrals are formed from the citrals. Whether attempting to isolate citral from a reaction solution of a petrochemical or an oxidation citral producing process, when distilling the citral from the reaction solution, the purity and yield of the citral desired will be adversely affected by the isomerization of the citral, resulting in the formation of the isocitrals. Similarly, when attempting to increase the purity of a crude citral solution, the problem of isocitral formation in the distillation process is also present.
A third factor which is encountered in the distillation of citral is citral sensitivity to pH. Citral, being an alpha-beta unsaturated terpene aldehyde, is known to be sensitive to both acidic and basic conditions, as well as temperatures above ambient. Under these conditions citral undergoes a number of reactions which result in not only its loss to residue, but also formation of undesirable components. If the pH of the citral is too low, low citral yields due to high loss of citral to residue during the distillation process can be experienced. Residue as used herein refers to compounds formed from the citral by mechanisms such as dimerization, polymerization, or condensation, for example. These higher molecular weight compounds have higher boiling points than the citral, and as such remain in the kettle during the distillation of the citral from the mixture, thereby decreasing the citral yield. As reported in "A Fresh Look at Citral," by D. H. Kingston, (Manufacturing Chemist, Dec. 1962, page 512) citral is very reactive chemically and decomposes readily in the presence of acids or alkali. Kingston reports that paracymene is formed from the citral in the presence of strong acids, and that even under mild acidic conditions structures such as terpineol and terpinolene, and alpha- or beta-cyclocitrals may be formed.
For these reasons, most producers of citral require that the citral remain at a pH of 7 or above. In fact, most of the work performed prior to the surprising discovery of this invention involved the use of a calcium carbonate buffer to ensure that the citral remained at a pH of 7 or slightly higher.
Thus, while fractional distillation may be preferred over adsorptive techniques to separate or purify citral, there is a need to develop a method for fractional distillation which minimizes the production of isocitrals from the citral mixture during the distillation process. At the same time, the method must not reduce the yield of citral through degradation or formation of other compounds.